In recent years, Copper (Cu) is commonly used in semiconductor integrated circuits for interconnections because it has better conductivity and is more reliable than other metals such as aluminum and aluminum alloys. However, mechanical stress still remains a technical challenge. The difference in thermal expansion coefficient between a copper conductor and a silicon substrate is a typical cause to mechanical stress. For example, copper expands seventeen parts-per-million per degree Celsius (C), and silicon expands three parts per million per degree C. For a three-inch (about one-tenth meter) long copper conductor, the difference in expansion between the copper conductor and the silicon substrate is 1.4 microns per degree C. For a one hundred degree Celsius temperature variation, the difference in expansion between the copper conductor and the silicon substrate is one hundred and forty microns. This significant difference in expansion leads to severe mechanical stress and is likely to cause the copper conductor to break.
FIG. 1 (Prior Art) illustrates a simplified top-down view of a silicon substrate 1. Silicon substrate 1 includes a copper conductor 2 and a copper conductor 3. Copper conductor 2 is three inches long and connects pads 4 and 5. Copper conductor 3 is also three inches long and connects pads 6 and 7. As illustrated in FIG. 1, copper conductor 2 is straight and copper conductor 3 bends in the middle. When substrate 1 is temperature cycled over a range from zero to seventy degrees Celsius (the commercial temperature range), both copper conductors 2 and 3 would expand (when temperature increases) or contract (when temperature decreases) up to one hundred and twenty microns. However, pads 4-7 are fixed to the silicon substrate and expand or contract up to twenty microns. As a result, a tension break may occur in copper conductor 2 and a compression break may occur in copper conductor 3.
In the current semiconductor market, the size of an integrated circuit is in general much smaller than three inches on a side. For example, the largest Field Programmable Gate Array (FPGA) chip today is about one inch long on each side of the chip. For a copper conductor that is shorter than one inch, mechanical stress is usually not severe enough to cause the copper conductor to break. As a result, little effort has been directed to addressing the mechanical stress issue. However, in a large area of power and ground planes, long interconnect wires are preferred. Therefore, it is desirable to be able to fabricate a long signal conductor that is reliable and will not easily break due to temperature variations.